Chapter 11
Paradise Lost—π-Electron Conjugation
in Homologs and Derivatives of Perylene
Ivan Gutman and Slavko Radenković
Abstract Various Kekulé–structure–based models, aimed at describing π-electron
conjugation in polycyclic aromatic compounds are briefly described. Our main
concern are benzenoid hydrocarbons, π-electron systems in which the Kekulé–
structure–based approaches are expected to yield the best results. Although there
are numerous examples in which reasonings based on Kekulé structures render
correct results, there exist cases in which significant violations are encountered.
Perylene, its homologs, and derivatives are characteristic representatives of such
“anomalous” conjugated systems. Violations from the predictions of the Kekulé–
structure–based models are verified by means of a variety of Kekulé–structure–
independent theoretical methods.
11.1 Introduction
The way how August Kekulé discovered the structural formula of benzene is one of
the legends of the history of chemistry [1, 2]. In 1865/6, Kekulé proposed for
benzene the hexagonal formula 1 shown in Fig. 11.1, according to which the carbon
atoms would be three-valent (or, in more modern terminology: three-coordinate). In
order to make the carbon atoms four-valent, in 1872 Kekulé inserted three double
bonds into his benzene formula, which can be done in two distinct ways—formulas
2 and 3 shown in Fig. 11.1. Nowadays, these are referred to as the Kekulé structures
or the Kekulé structural formulas of benzene.
It soon became evident that not only benzene, but the whole class of polycyclic
aromatic compounds (benzenoid hydrocarbons in particular), possess a non-unique
I. Gutman (&) Á S. Radenković
Faculty of Science, University of Kragujevac, Kragujevac, Serbia
e-mail: gutman@kg.ac.rs
S. Radenković
e-mail: slavko.radenkovic@gmail.com
I. Gutman
State University of Novi Pazar, Novi Pazar, Serbia
© Springer International Publishing Switzerland 2016
R. Chauvin et al. (eds.), Applications of Topological Methods
in Molecular Chemistry, Challenges and Advances in Computational
Chemistry and Physics 22, DOI 10.1007/978-3-319-29022-5_11
297
Paradise Lost—π-Electron Conjugation
in Homologs and Derivatives of Perylene
Ivan Gutman and Slavko Radenković
Abstract Various Kekulé–structure–based models, aimed at describing π-electron
conjugation in polycyclic aromatic compounds are briefly described. Our main
concern are benzenoid hydrocarbons, π-electron systems in which the Kekulé–
structure–based approaches are expected to yield the best results. Although there
are numerous examples in which reasonings based on Kekulé structures render
correct results, there exist cases in which significant violations are encountered.
Perylene, its homologs, and derivatives are characteristic representatives of such
“anomalous” conjugated systems. Violations from the predictions of the Kekulé–
structure–based models are verified by means of a variety of Kekulé–structure–
independent theoretical methods.
11.1 Introduction
The way how August Kekulé discovered the structural formula of benzene is one of
the legends of the history of chemistry [1, 2]. In 1865/6, Kekulé proposed for
benzene the hexagonal formula 1 shown in Fig. 11.1, according to which the carbon
atoms would be three-valent (or, in more modern terminology: three-coordinate). In
order to make the carbon atoms four-valent, in 1872 Kekulé inserted three double
bonds into his benzene formula, which can be done in two distinct ways—formulas
2 and 3 shown in Fig. 11.1. Nowadays, these are referred to as the Kekulé structures
or the Kekulé structural formulas of benzene.
It soon became evident that not only benzene, but the whole class of polycyclic
aromatic compounds (benzenoid hydrocarbons in particular), possess a non-unique
I. Gutman (&) Á S. Radenković
Faculty of Science, University of Kragujevac, Kragujevac, Serbia
e-mail: gutman@kg.ac.rs
S. Radenković
e-mail: slavko.radenkovic@gmail.com
I. Gutman
State University of Novi Pazar, Novi Pazar, Serbia
© Springer International Publishing Switzerland 2016
R. Chauvin et al. (eds.), Applications of Topological Methods
in Molecular Chemistry, Challenges and Advances in Computational
Chemistry and Physics 22, DOI 10.1007/978-3-319-29022-5_11
297